1. Field of the Invention
The present invention relates to a flat display device and, more particularly, to a flat display device capable of preventing variation in a threshold voltage of a driving transistor during rendering of a white image.
2. Discussion of the Related Art
Flat display devices in related art include liquid crystal displays (LCDs) and plasma displays (PDPs) which are thin, light, and portable and have high performance, and organic light emitting display devices which may eliminate disadvantages of heavy weight and bulky cathode ray tubes (CRTs).
Such flat display devices include unit pixels each constituted by a red sub-pixel, a green sub-pixel, and a blue sub-pixel, to display an image of various colors. Each unit pixel in flat display devices further includes a white sub-pixel, in addition to red, green, and blue sub-pixels. The white sub-pixel does not require a color filter and exhibits higher transmittance than the remaining sub-pixels. In this regard, an enhancement in efficiency is achieved. Such a flat display device, which includes a white sub-pixel, displays an image by transforming input data of three colors, namely, red, green, and blue, into data of four colors, namely, red, green, blue, and white. In particular, in conventional four-color flat display devices, a white image is displayed through a combination of two of the red, green, and blue sub-pixels and the white sub-pixel. When a white image is rendered for a lengthened period of time, threshold voltages of driving transistors of the non-driving sub-pixels (e.g., transistors connected to organic light emitting diodes in an organic light emitting display device or transistors connected to pixel electrodes in a liquid crystal display device) are shifted in the negative direction due to stress applied to the driving transistors (e.g., negative bias temperature illumination stress (NTBis)), as compared to those of the driving sub-pixels. To solve this problem, a data voltage may be shifted to compensate for the shifted levels of the threshold voltages at the outside. However, the range of compensation is limited and, as such, there is a limitation on compensation. In particular, when the threshold voltages are continuously shifted in the negative direction beyond the compensation range, luminance may be increased and, as such, degradation of reliability may occur.